1. 
 Anand, S., et al.
(författare)

Optical followup of the neutron star–black hole mergers S200105ae and S200115j
 2020

Ingår i: Nature Astronomy.  : Nature Research.  23973366.

Tidskriftsartikel (refereegranskat)abstract
 LIGO and Virgo’s third observing run revealed the first neutron star–black hole (NSBH) merger candidates in gravitational waves. These events are predicted to synthesize rprocess elements1,2 creating optical/nearinfrared ‘kilonova’ emission. The joint gravitational wave and electromagnetic detection of an NSBH merger could be used to constrain the equation of state of dense nuclear matter3, and independently measure the local expansion rate of the Universe4. Here, we present the optical followup and analysis of two of the only three highsignificance NSBH merger candidates detected to date, S200105ae and S200115j, with the Zwicky Transient Facility5. The Zwicky Transient Facility observed ~48% of S200105ae and ~22% of S200115j’s localization probabilities, with observations sensitive to kilonovae brighter than −17.5 mag fading at 0.5 mag d−1 in the g and rbands; extensive searches and systematic followup of candidates did not yield a viable counterpart. We present stateoftheart kilonova models tailored to NSBH systems that place constraints on the ejecta properties of these NSBH mergers. We show that with observed depths of apparent magnitude ~22 mag, attainable in metreclass, widefieldofview survey instruments, strong constraints on ejecta mass are possible, with the potential to rule out low mass ratios, high black hole spins and large neutron star radii.


2. 
 Broderick, J. W., et al.
(författare)

LOFAR 144MHz followup observations of GW170817
 2020

Ingår i: Monthly Notices of the Royal Astronomical Society.  : Oxford University Press (OUP).  00358711 . 13652966. ; 494:4, s. 51105117

Tidskriftsartikel (refereegranskat)abstract
 We present lowradiofrequency followup observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGOVirgo. These data, with a central frequency of 144 MHz, were obtained with LOFAR, the LowFrequency Array. The maximum elevation of the target is just 13 degrees.7 when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On timescales of 130138 and 371374 d after the merger event, we obtain 3s upper limits for the afterglow component of 6.6 and 19.5mJy beam(1), respectively. Using our best upper limit and previously published, contemporaneous higher frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144 MHz: the twopoint spectral index alpha(610)(144) greater than or similar to 2.5. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.


3. 
 Kasliwal, Mansi M., et al.
(författare)

Kilonova Luminosity Function Constraints Based on Zwicky Transient Facility Searches for 13 Neutron Star Merger Triggers during O3
 2020

Ingår i: Astrophysical Journal.  : American Astronomical Society.  0004637X . 15384357. ; 905:2

Tidskriftsartikel (refereegranskat)abstract
 We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo's third observing run (O3). We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook followup with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization area of 4480 deg(2), a median distance of 267 Mpc, and falsealarm rates ranging from 1.5 to 10(25) yr(1). The ZTF coverage in the g and r bands had a median enclosed probability of 39%, median depth of 20.8 mag, and median time lag between merger and the start of observations of 1.5 hr. The O3 followup by the GROWTH team comprised 340 UltraViolet/Optical/InfraRed (UVOIR) photometric points, 64 OIR spectra, and three radio images using 17 different telescopes. We find no promising kilonovae (radioactivitypowered counterparts), and we show how to convert the upper limits to constrain the underlying kilonova luminosity function. Initially, we assume that all GW triggers are bona fide astrophysical events regardless of falsealarm rate and that kilonovae accompanying BNS and NSBH mergers are drawn from a common population; later, we relax these assumptions. Assuming that all kilonovae are at least as luminous as the discovery magnitude of GW170817 (16.1 mag), we calculate that our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than 16.6 mag (the extrapolated peak magnitude of GW170817) and fade at a rate of 1 mag day(1) (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations based on the online classifications, the joint probability of zero detections, assuming all kilonovae are brighter than 16.6 mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, no more than <57% (<89%) of putative kilonovae could be brighter than 16.6 mag assuming flat evolution (fading by 1 mag day(1)) at the 90% confidence level. If we further take into account the online terrestrial probability for each GW trigger, we find that no more than <68% of putative kilonovae could be brighter than 16.6 mag. Comparing to model grids, we find that some kilonovae must have Mej M, Xlan > 10(4), or > 30 degrees to be consistent with our limits. We look forward to searches in the fourth GW observing run; even 17 neutron star mergers with only 50% coverage to a depth of 16 mag would constrain the maximum fraction of bright kilonovae to <25%.


4. 
 Andreoni, Igor, et al.
(författare)

GROWTH on S190814bv : Deep Synoptic Limits on the Optical/Nearinfrared Counterpart to a Neutron StarBlack Hole Merger
 2020

Ingår i: Astrophysical Journal.  : American Astronomical Society.  0004637X . 15384357. ; 890:2

Tidskriftsartikel (refereegranskat)abstract
 On 2019 August 14, the Advanced LIGO and Virgo interferometers detected the highsignificance gravitational wave (GW) signal S190814bv. The GW data indicated that the event resulted from a neutron starblack hole (NSBH) merger, or potentially a lowmass binary BH merger. Due to the low falsealarm rate and the precise localization (23 deg(2) at 90%), S190814bv presented the community with the best opportunity yet to directly observe an optical/nearinfrared counterpart to an NSBH merger. To search for potential counterparts, the GROWTH Collaboration performed realtime image subtraction on six nights of public Dark Energy Camera images acquired in the 3 weeks following the merger, covering >98% of the localization probability. Using a worldwide network of followup facilities, we systematically undertook spectroscopy and imaging of optical counterpart candidates. Combining these data with a photometric redshift catalog, we ruled out each candidate as the counterpart to S190814bv and placed deep, uniform limits on the optical emission associated with S190814bv. For the nearest consistent GW distance, radiative transfer simulations of NSBH mergers constrain the ejecta mass of S190814bv to be Mej < 0.04 Mcircle dot at polar viewing angles, or Mej < 0.03 Mcircle dot if the opacity is kappa < 2 cm(2)g(1). Assuming a tidal deformability for the NS at the high end of the range compatible with GW170817 results, our limits would constrain the BH spin component aligned with the orbital momentum to be chi < 0.7 for mass ratios Q < 6, with weaker constraints for more compact NSs.

